Abstract. We report properties of marine aerosol and clouds measured in
the shipping lanes between Monterey Bay and San Francisco off
the coast of Central California. Using a suite of aerosol
instrumentation onboard the CIRPAS Twin Otter aircraft, these
measurements represent a unique set of data contrasting the
properties of clean and ship-impacted marine air masses in dry
aerosol and cloud droplet residuals. Below-cloud aerosol exhibited
average mass and number concentrations of 2 μg m−3 and 510 cm−3, respectively, which are
consistent with previous studies performed off the coast of
California. Enhancements in vanadium and cloud droplet number
concentrations are observed concurrently with a decrease in cloud
water pH, suggesting that periods of high aerosol loading are
primarily linked to increased ship influence. Mass spectra
from a compact time-of-flight Aerodyne aerosol mass
spectrometer reveal an enhancement in the fraction of organic
at m/z 42 (f42) and 99 (f99) in
ship-impacted clouds. These ions are well correlated to each
other (R2>0.64) both in and out of cloud and constitute 14% (f44) and 3% (f99) of
organic mass during periods of enhanced
sulfate. High-resolution mass spectral analysis of these
masses from ship measurements suggests that the ions
responsible for this variation were oxidized, possibly due to
cloud processing. We propose that the organic fractions of
these ions be used as a metric for determining the extent to
which cloud-processed ship emissions impact the marine atmosphere where
(f42 > 0.15; f99 > 0.04) would imply heavy
influence from shipping emissions,
(0.05 < f42 < 0.15;
0.01 < f99 < 0.04) would imply moderate, but
persistent, influences from ships, and (f42 < 0.05;
f99 < 0.01) would imply clean, non-ship-influenced
air.